Spin MOSFET and reconfigurable logic circuit

ABSTRACT

A spin MOSFET includes: a first ferromagnetic layer provided on an upper face of a semiconductor substrate, and having a fixed magnetization direction perpendicular to a film plane; a semiconductor layer provided on an upper face of the first ferromagnetic layer, including a lower face opposed to the upper face of the first ferromagnetic layer, an upper face opposed to the lower face, and side faces different from the lower face and the upper face; a second ferromagnetic layer provided on the upper face of the semiconductor layer, and having a variable magnetization direction perpendicular to a film plane; a first tunnel barrier provided on an upper face of the second ferromagnetic layer; a third ferromagnetic layer provided on an upper face of the first tunnel barrier; a gate insulating film provided on the side faces of the semiconductor layer; and a gate electrode provided on the side faces of the semiconductor layer with the gate insulating film being interposed therebetween.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2009-74785 filed on Mar. 25, 2009in Japan, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spin MOSFET and a reconfigurablelogic circuit.

2. Related Art

In recent years, MRAM (Magnetic Random Access Memory) with a MTJ(Magnetic Tunnel Junction) having a tunnel barrier layer interposedbetween two vertical magnetization films has been drawing attention, forits lower spin injection current density and excellent thermalstability. In this specification, a “vertical magnetization film” is aferromagnetic film that has a magnetization direction (a magnetizationeasy-axis direction) substantially perpendicular to its upper face.

Meanwhile, intensive research and development are being made withrespect to devices having novel functions such as spin MOSFETs. One ofthose devices is a spin MOSFET having source/drain regions made of amagnetic material. A spin MOSFET is characteristically capable ofcontrolling its output properties simply by reversing the direction ofthe spin moment of the magnetic material of the source/drain regions. Bytaking advantage of this feature, it is possible to form a spin MOSFEThaving a reconfigurable function and an amplifying function, or areconfigurable logic circuit (see APL84 (2004) 2307, for example).

As a spin torque transfer switching method, a write method involvingspin injection has been suggested. A spin torque transfer switching canbe seen by spin-injecting spin-polarized current. Also, to apply spininjection writing to spin MOSFETs, a structure having a MTJ in one ofthe magnetic bodies forming the source/drain regions has been suggested(see JP-A 2008-66596 (KOKAI)). With the use of the structure disclosedin JP-A 2008-66596 (KOKAI), the following effects can be achieved:

1) Two outputs can be used for reading; and

2) A spin torque transfer switching can be utilized, since a MTJ (aferromagnetic stacked film) is provided.

Also, the use of vertical magnetization films in the MTJ in thesource/drain region in a spin MOSFET has been suggested by the inventor,and has been filed for a patent by the applicant (Japanese PatentApplication No. 2008-191146). In this spin MOSFET, source/drain regionsare provided at a distance from each other in a semiconductor substrate,and a gate electrode is formed on a semiconductor region to be thechannel region between the source region and the drain region. Aferromagnetic stacked film having a vertical magnetization film isprovided on each of the source/drain regions, and at least one of theferromagnetic stacked films on the source/drain regions has a MTJstructure (hereinafter also referred to as a lateral structure). Withthis structure, the spin-injection write current density can be lowered,the areas of the source/drain portions can be reduced, and thermalstability can be advantageously secured in a spin MOSFET.

In a case where vertical magnetization films are used in the MTJ of thesource or drain region, however, a leakage magnetic field generated fromthe fixed magnetization layer of the MTJ affects the magnetic recordinglayer, and shift adjustment cannot be performed, as will be describedbelow. Such a leakage magnetic field also affects adjacent spin MOSFETs.In such a spin MOSFET having a lateral structure including verticalmagnetization films, spin relaxation in the semiconductor region to bethe channel region is accelerated due to the leakage magnetic field.This problem was not recognized in the past, and was recognized first bythe inventor.

As described above, in a spin MOSFET using vertical magnetization filmsin the ferromagnetic material of the MTJ in the source/drain region, thespin-injection write current density becomes lower, and excellentthermal stability and the likes can be achieved. However, a spin MOSFEThaving a lateral structure has the following problems: difficult fieldshift adjustment, influence of a leakage magnetic field on adjacenttransistors, and accelerated spin relaxation in the semiconductor regionto be the channel region due to the leakage magnetic field.

SUMMARY OF THE INVENTION

The present invention has been made in view of these circumstances, andan object thereof is to provide a spin MOSFET that can reduce theinfluence of a leakage magnetic field on adjacent transistors, enableshift adjustment, and restrain spin relaxation in the channel region,even where vertical magnetization films are used in the ferromagneticmaterial of the MTJ in the source/drain region. The present inventionalso aims to provide a reconfigurable logic circuit.

A spin MOSFET according to a first aspect of the present inventionincludes: a first ferromagnetic layer provided on an upper face of asemiconductor substrate, and having a fixed magnetization directionperpendicular to a film plane; a semiconductor layer provided on anupper face of the first ferromagnetic layer, including a lower faceopposed to the upper face of the first ferromagnetic layer, an upperface opposed to the lower face, and side faces different from the lowerface and the upper face, and serving as a channel; a secondferromagnetic layer provided on the upper face of the semiconductorlayer, and having a variable magnetization direction perpendicular to afilm plane; a first tunnel barrier provided on an upper face of thesecond ferromagnetic layer; a third ferromagnetic layer provided on anupper face of the first tunnel barrier; a gate insulating film providedon the side faces of the semiconductor layer; and a gate electrodeprovided on the side faces of the semiconductor layer with the gateinsulating film being interposed therebetween.

A spin MOSFET according to a second aspect of the present inventionincludes: a first ferromagnetic layer formed on an upper face of asemiconductor substrate; a first tunnel barrier provided on an upperface of the first ferromagnetic layer; a second ferromagnetic layerprovided on an upper face of the first tunnel barrier, and having avariable magnetization direction perpendicular to a film plane; asemiconductor layer provided on an upper face of the secondferromagnetic layer, including a lower face opposed to the upper face ofthe second ferromagnetic layer, an upper face opposed to the lower face,and side faces different from the lower face and the upper face, andserving as a channel; a third ferromagnetic layer provided on the upperface of the semiconductor layer, and having a fixed magnetizationdirection perpendicular to a film plane; a gate insulating film providedon the side faces of the semiconductor layer; and a gate electrodeprovided on the side faces of the semiconductor layer with the gateinsulating film being interposed.

A spin MOSFET according to a third aspect of the present inventionincludes: a source unit and a drain unit provided at a distance fromeach other on a semiconductor substrate, one of the source unit and thedrain unit comprising a first ferromagnetic stacked film that includes afirst ferromagnetic layer having a fixed magnetization directionperpendicular to a film plane, the other one of the source unit and thedrain unit comprising a second ferromagnetic stacked film that includesa second ferromagnetic layer having a variable magnetization directionperpendicular to a film plane, a first tunnel barrier provided on thesecond ferromagnetic layer, and a third ferromagnetic layer provided onthe first tunnel barrier and having a fixed magnetization directionperpendicular to a film plane and antiparallel to the magnetizationdirection of the first ferromagnetic layer; a gate insulating filmprovided on a portion of the semiconductor substrate, the portion beinglocated between the source unit and the drain unit; a gate electrodeprovided on the gate insulating film; a first hard bias film providedabove the gate electrode; a second hard bias film provided above aportion of the semiconductor substrate, the portion being located on afirst side of the source unit, the first side being opposite to a secondside on which the first hard bias film is disposed; and a third hardbias film provided above a portion of the semiconductor substrate, theportion being located on a third side of the drain unit, the third sidebeing opposite to a fourth side on which the first hard bias film isdisposed, magnetization directions of the first through third hard biasfilms being parallel to the magnetization direction of the thirdferromagnetic layer.

A spin MOSFET according to a fourth aspect of the present inventionincludes: a source unit and a drain unit provided at a distance fromeach other on a semiconductor substrate, one of the source unit and thedrain unit comprising a first ferromagnetic stacked film that includes afirst ferromagnetic layer having a fixed magnetization directionperpendicular to a film plane, the other one of the source unit and thedrain unit comprising a second ferromagnetic stacked film that includesa second ferromagnetic layer having a variable magnetization directionperpendicular to a film plane, a first tunnel barrier provided on thesecond ferromagnetic layer, and a third ferromagnetic layer provided onthe first tunnel barrier and having a fixed magnetization directionperpendicular to a film plane and antiparallel to the magnetizationdirection of the first ferromagnetic layer; a gate insulating filmprovided on a portion of the semiconductor substrate, the portion beinglocated between the source unit and the drain unit; a gate electrodeprovided on the gate insulating film; a first hard bias film provided onthe gate electrode; a second hard bias film provided on the source unit;and a third hard bias film provided on the drain unit, magnetizationdirections of the first through third hard bias films beingsubstantially antiparallel to the magnetization direction of the thirdferromagnetic layer.

A reconfigurable logic circuit according to a fifth aspect of thepresent invention includes: two field effect transistors, at least oneof the two field effect transistors being the spin MOSFET according tothe first aspect, the two field effect transistors having a commonfloating gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a spin MOSFET according to a firstembodiment of the present invention;

FIG. 2 is a cross-sectional view of a spin MOSFET according to a secondembodiment of the present invention;

FIG. 3 is a cross-sectional view of a spin MOSFET according to a thirdembodiment of the present invention;

FIG. 4 shows a leakage magnetic field of a MTJ that has fixedmagnetization layers with in-plane magnetization and is a syntheticstructure;

FIG. 5 shows a leakage magnetic field of a MTJ that has fixedmagnetization layers with vertical magnetization and is a syntheticstructure;

FIG. 6 shows the channel length dependence of the spin diffusion lengthin the channel region;

FIG. 7 is a cross-sectional view of a spin MOSFET according to a fourthembodiment;

FIG. 8( a) is a diagram for explaining an in-plane shape of a MTJ thateasily has a magnetization reversal at the time of spin injection;

FIG. 8( b) is a diagram for explaining an in-plane shape of a MTJ thatdoes not easily have a magnetization reversal at the time of spininjection;

FIG. 9 is a cross-sectional view of a spin MOSFET according to a fifthembodiment;

FIG. 10 is a cross-sectional view of a spin MOSFET according to a sixthembodiment of the present invention;

FIG. 11 is a cross-sectional view of a spin MOSFET according to aseventh embodiment of the present invention;

FIG. 12 is a cross-sectional view of a spin MOSFET according to aneighth embodiment of the present invention;

FIG. 13 is a cross-sectional view of a spin MOSFET according to a ninthembodiment of the present invention;

FIG. 14 is a cross-sectional view of a spin MOSFET according to a tenthembodiment of the present invention;

FIG. 15 is a cross-sectional view of a spin MOSFET according to aneleventh embodiment of the present invention;

FIG. 16 is a cross-sectional view of a spin MOSFET according to atwelfth embodiment of the present invention;

FIG. 17 is a cross-sectional view for explaining a manufacture methodaccording to Example 1;

FIGS. 18( a), 18(b) show the results of measurement of the rate ofresistance change and the shift amount in Example 1 and ComparativeExample 1 respectively;

FIGS. 19( a), 19(b) show the results of measurement of the rate ofresistance change and the shift amount in Example 2 and ComparativeExample 1 respectively;

FIG. 20 shows the channel length dependence of the spin diffusion lengthin the channel region in Example 4;

FIG. 21 shows a logic circuit according to a thirteenth embodiment;

FIG. 22 shows the floating-gate voltage dependence of the output of thelogic circuit of the thirteenth embodiment;

FIG. 23 shows a logical table in a case where the logic circuit of thethirteenth embodiment functions as an AND circuit;

FIG. 24 shows a logical table in a case where the logic circuit of thethirteenth embodiment functions as an OR circuit;

FIG. 25 shows a logic circuit according to a first modification of thethirteenth embodiment;

FIG. 26 shows a logic circuit according to a second modification of thethirteenth embodiment;

FIG. 27 is a cross-sectional view of a ferromagnetic layer having asynthetic structure in which ferromagnetic films are stacked via anonmagnetic film; and

FIG. 28 is a cross-sectional view of a spin MOSFET according to amodification of the first embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The following is a description of embodiments of the present invention,with reference to the accompanying drawings. Those drawings are merelyschematic, and the sizes of the components and the relative sizes of thecomponents differ from those in reality. Also, the sizes or proportionsof the same components might vary among the drawings.

First Embodiment

FIG. 1 shows a spin MOSFET according to a first embodiment of thepresent invention. The spin MOSFET of this embodiment is a vertical typespin MOSFET, and is formed on a SOI substrate 60 that includes asupporting substrate 61 made of Si, a buried oxide film 62, and a SOI(Silicon On Insulator) layer 63. The spin MOSFET may be formed on a Sibulk substrate, instead of a SOI substrate. A base layer 65 is formed onthe SOI layer 63. A ferromagnetic layer 72 that has magnetizationsubstantially perpendicular to a film plane and has a fixedmagnetization direction is formed on the base layer 65. In thisspecification, a “film plane” of a ferromagnetic layer is an upper faceof the ferromagnetic layer. A channel layer 74 that is formed fromp-type semiconductor crystals is formed on the ferromagnetic layer 72. Afree layer 78 that has a variable magnetization direction and is formedof a ferromagnetic layer is formed on the channel layer 74. A tunnelbarrier 80 is formed on the free layer 78, and a ferromagnetic layer 82having vertical magnetization is formed on the tunnel barrier 80. Asshown in FIG. 27, this ferromagnetic layer 82 has a synthetic structurein which ferromagnetic films 82 a, 82 c having vertical magnetizationare stacked via a nonmagnetic film 82 b, or a stacked structure formedof a first ferromagnetic film 82 a, a nonmagnetic film 82 b, and asecond ferromagnetic film 82 c. The first ferromagnetic film 82 a andthe second ferromagnetic film 82 c each have a magnetization directionfixed (invariable) in a direction substantially perpendicular to thefilm plane, and are antiferromagnetically coupled via the nonmagneticfilm 82 b. In other words, the ferromagnetic layer 82 is a fixedmagnetization layer. In this embodiment, the free layer 78, the tunnelbarrier 80, and the ferromagnetic layer 82 forms a ferromagnetic stackedfilm MTJ. A nonmagnetic metal layer 84 is formed on the ferromagneticlayer 82. Instead of the nonmagnetic metal layer 84, anantiferromagnetic layer may be used. In such a case, the magnetizationof the ferromagnetic layer 82 is stabilized. Alternatively, theferromagnetic layer 72 may have a synthetic structure in whichferromagnetic films having vertical magnetization are stacked, with anonmagnetic film in between. In this embodiment, the magnetizationdirections of the ferromagnetic layer 72 and the ferromagnetic film ofthe ferromagnetic layer 82 closest to the channel layer 74 aresubstantially antiparallel to each other. The free layer 78 may have asynthetic structure in which ferromagnetic films are stacked, with anonmagnetic film being interposed in between.

As described above, in this embodiment, a stacked structure that isformed of the ferromagnetic layer 72, the channel layer 74 formed fromsemiconductor crystals, the free layer 78, the tunnel barrier 80, theferromagnetic layer 82, and the nonmagnetic layer 84, is formed on thebase layer 65. In this stacked structure, as shown in FIG. 28, thelayers may be stacked in reverse order, except for the nonmagnetic metallayer 84. More specifically, the stacked structure formed on the baselayer 65 may be formed by stacking the ferromagnetic layer 82, thetunnel barrier 80, the free layer 78, the channel layer 74 formed fromsemiconductor crystals, the ferromagnetic layer 72, and the nonmagneticmetal layer 84 in this order. A gate electrode 76 is formed to surroundthe channel layer 74 of this stacked structure, with a gate insulatingfilm 90 a being interposed therebetween. Incidentally, the gateinsulating film 90 a may be provided on a part of the side faces of thesemiconductor layer, and the gate electrode 76 may be provided on theside faces of the semiconductor layer with the gate insulating filmbeing interposed therebetween. An insulating film 90 is further providedbetween the gate electrode 90 and the base layer 65.

In this embodiment having the above structure, a synthetic structure isused as the fixed magnetization layer 82. The fixed magnetization layer82 is separated from the ferromagnetic layer 72 by the channel layer 74,and the gate electrode 76 is provided on the side faces. Accordingly,the shift of the free layer 78 located below the fixed magnetizationlayer 82 via the tunnel barrier 80 can be adjusted. Also, in a casewhere adjacent cells are closely arranged, influence of the leakagemagnetic filed can be eliminated. Since the spin MOSFET of thisembodiment is a vertical structure and has the ferromagnetic layer 82with a magnetization direction antiparallel to the magnetizationdirection of the ferromagnetic layer 72, the lines of magnetic forcegenerated from the ferromagnetic layer 82 (the magnetic force indicatedby solid lines in FIG. 1) are opposite to the lines of magnetic forcegenerated from the ferromagnetic layer 72 (the magnetic force not shownin FIG. 1), and the leakage magnetic field is substantially cancelledaccordingly. Even if the leakage magnetic field is not cancelled, thedirection of spin-polarized electrons flowing in the channel layer 74 issubstantially parallel or antiparallel to the directions of the magneticforce generated from the ferromagnetic layer 72 and the ferromagneticlayer 82. Accordingly, the spin-polarized electrons are hardly affectedby the leakage magnetic field. Thus, the channel layer 74 is hardlyaffected by the leakage magnetic field, and spin relaxation can berestrained.

In this embodiment, source/drain regions doped with n-type impurities athigh density should preferably be formed in the interfacial sides of theferromagnetic layer 72 and the free layer 78 in the side of the channellayer 74.

The gate insulating film 90 a may be a SiO₂ film used in conventionalMOSFET, or may be made of a high-dielectric material formed by melting ametal such as Zr, Hf, or La in SiO₂ such as Hf silicate or Zr silicate.P-type-impurities-doped or n-type-impurities-doped poly-Si or poly-SiGeis used as the gate electrode 76. In a case where the poly-Si orpoly-SiGe is combined with a high-dielectric insulating film, it ispossible to use a metal-based material such as TiN, TaN, TaC, arare-earth metal, or a rare-earth transition metal alloy.

Since a MTJ formed of the free layer 78, the tunnel barrier 80, and theferromagnetic layer 82 is provided in this embodiment, spin injectionwriting can be preformed. In spin injection writing, the magnetizationdirection of the free layer 78 can be changed by switching the directionof the current flow between an upward direction and a downward directionin the structure shown in FIG. 1.

As described above, in accordance with this embodiment, influence of aleakage magnetic field on adjacent transistors can be eliminated, andshift adjustment can be performed.

Second Embodiment

FIG. 2 is a spin MOSFET according to a second embodiment of the presentinvention. The spin MOSFET of this embodiment is a vertical spin MOSFET,and is the same as the spin MOSFET of the first embodiment shown in FIG.1, except that a tunnel barrier 73 is provided between the channel layer74 formed from semiconductor crystals and the ferromagnetic layer 72,and a tunnel barrier 77 provided between the channel layer 74 and thefree layer 78. If tunnel barriers made of a material such as MgO thatcan epitaxially grow even if the channel layer 74 is made of Ge or GaAsare used as the tunnel barriers 73 and 77, the tunnel barriers 73 and 77can also serve as diffusion barriers, and advantageously improve theresistance to device pressure. In this embodiment, a stacked structurethat is formed of the ferromagnetic layer 72, the tunnel barrier 73, thechannel layer 74 formed from semiconductor crystals, the tunnel barrier77, the free layer 78, the tunnel barrier 80, the ferromagnetic layer82, and the nonmagnetic layer 84, is formed on the base layer 65. Inthis stacked structure, the layers may be stacked in reverse order,except for the nonmagnetic metal layer 84, as in the first embodiment.More specifically, the stacked structure formed on the base layer 65 maybe formed by stacking the ferromagnetic layer 82, the tunnel barrier 80,the free layer 78, the tunnel barrier 77, the channel layer 74 formedfrom semiconductor crystals, the tunnel barrier 73, the ferromagneticlayer 72, and the nonmagnetic metal layer 84 in this order.

In this embodiment, influence of a leakage magnetic field on adjacenttransistors can be eliminated as in the first embodiment. Accordingly,spin relaxation can be restrained, and shift adjustment can beperformed.

In the first and the second embodiments, the uppermost SOI layer may bea Si crystal layer or a SiGe crystal layer. As for the material of thesemiconductor channel layer, it is possible to select a material such asSi, SiGe, GaAs, or InGaAs, instead of Ge. In such a case, it isessential to take into consideration the influence of the crystallattice of the lowest SOI layer 63 on the lattice spacing in themagnetic layer 72 and the channel layer 74. Particularly, the latticeand distortion in the channel layer 74 has large influence on themobility of the carriers in the channel layer 74. It is preferable tomatch the lattice spacing in the magnetic layer 72 with the lattice inthe base layer 65. However, the lattice of the semiconductor crystals inthe channel layer 74, which is slightly different from the crystallattice in the base layer 65, is selected, and distortion is activelycaused. In this manner, the channel mobility can be increased. Forexample, SiGe crystals with 80% Ge composition are arranged to form thebase layer 65, and the magnetic layer 72 that lattice-matches thecrystals is stacked. A Ge crystal layer as the channel layer 74 is thenstacked. The Ge crystal lattice of the channel layer 74 becomes slightlylarger than the lattice of the SiGe crystal layer. Therefore, thecrystal lattice of the channel layer 74 contracts due to compressiondistortion applied in a transverse direction in conformity to the baselayer 65, and expands due to tensile stress applied in a verticaldirection. In this manner, the tensile stress is applied in the currentflowing direction of the spin MOSFET, and the electron mobility iseffectively increased.

Third Embodiment

FIG. 3 is a cross-sectional view of a spin MOSFET according to a thirdembodiment of the present invention. The spin MOSFET of this embodimentis a lateral n-type spin MOSFET that is formed in a device region 3 of ap-type semiconductor substrate 2. This device region 3 is asemiconductor region divided by device-insolating insulating films 4.The semiconductor region may be part of a semiconductor substrate, ormay be a well region formed on a semiconductor substrate. Alternatively,the semiconductor region may be a SOI layer on a SOI substrate. In thisspecification, the device region 3 may be a semiconductor region that ispart of a p-type semiconductor substrate, or may be a p-well regionformed on an n-type substrate. In this device region 3, an n-typeimpurity diffusion region 6 a and an n-type impurity diffusion region 6b are formed at a distance from each other. An n⁺-impurity diffusionregion 7 a and an n⁺-type impurity diffusion region 7 b having higherdensity than the n-type impurity diffusion region 6 a and the n-typeimpurity diffusion region 6 b are formed on the surfaces of the n-typeimpurity diffusion region 6 a and the n-type impurity diffusion region 6b, respectively. The n-type impurity diffusion region 6 a and then⁺-impurity diffusion region 7 a form a source region 5 a, and then-type impurity diffusion region 6 b and the n⁺-type impurity diffusionregion 7 b form a drain region 5 b.

A gate insulating film 9 is provided on a portion of the semiconductorsubstrate 2 that is to serve as a channel region 8 located between thesource region 5 a and the drain region 5 b. A gate 10 made of anonmagnetic metal is placed on the gate insulating film 9, for example.A source unit 15 a is formed on the source region 5 a, with a tunnelbarrier 14 a being interposed in between. A drain unit 15 b is formed onthe drain region 5 b, with a tunnel barrier 14 b being interposed inbetween. The source unit 15 a is formed of a ferromagnetic layer 18 a.The drain unit 15 b has a ferromagnetic stacked structure (aferromagnetic stacked film) in which a ferromagnetic layer 16 b, anonmagnetic layer 17 b, and a ferromagnetic layer 18 b are stacked inthis order. In a case where the nonmagnetic layer 17 b is a tunnelbarrier, the drain unit 15 b is a ferromagnetic tunnel junction (MTJ(Magnetic Tunnel Junction). In this specification, the ferromagneticstacked film is also referred to as the MTJ stacked film. In thisembodiment, the drain unit 15 b has a MTJ structure, and the source unit15 a is formed of a ferromagnetic layer. However, the source unit 15 amay have a MTJ structure, and the drain unit 15 b may be formed of aferromagnetic layer.

In this embodiment, the ferromagnetic layer 18 a of the source unit 15 ais a fixed magnetization layer that has a fixed (invariable)magnetization direction. The ferromagnetic layer 16 b of the drain unit15 b closer to the semiconductor substrate 2 is a free layer (a magneticrecording layer) having a variable magnetization direction. The otherferromagnetic layer 18 b of the drain unit 15 b, which is farther fromthe semiconductor substrate 2, is a fixed magnetization layer. In thisembodiment, the magnetization direction (the magnetization easy-axisdirection) of each of the ferromagnetic layers 16 b, 18 a, and 18 b issubstantially perpendicular to the film plane. In this specification,the “film plane” is the upper face of the stacked film.

A nonmagnetic metal layer 20 a and a nonmagnetic metal layer 20 b areplaced on the source unit 15 a and the drain unit 15 b, respectively.The source unit 15 a, the nonmagnetic metal layer 20 a, and the gate 10are insulated from one another by a gate side wall 12 made of aninsulating material. The drain unit 15 b, the nonmagnetic metal layer 20b, and the gate 10 are insulated from one another by a gate side wall 12made of an insulating material. The nonmagnetic metal layer 20 a isconnected to a wiring 42 a via a plug 40 a, and the nonmagnetic metallayer 20 b is connected to a wiring 42 b via a plug 40 b.

In the spin MOSFET having the above structure, the spin injection writecurrent density can be advantageously lowered, the areas of thesource/drain units can be advantageously made smaller, and thermalstability can be achieved.

However, the fixed magnetization layer 18 a of the source unit 15 a hasa leakage magnetic field. Therefore, the spin-polarized electronsflowing in the channel 8 are adversely affected, and the adjacent spinMOSFET on the left (not shown) is also adversely affected in the caseillustrated in FIG. 3. If vertical magnetization films are used in theMTJ of the drain unit 15 b, the leakage magnetic field from the fixedmagnetization layer 18 b of the MTJ adversely affects the free layer 16b, to hinder shift adjustment. Also, the adjacent spin MOSFET isadversely affected.

In a case where magnetic films having magnetization substantiallyparallel to the film plane are used for the MTJ, a syntheticantiferromagnetic stacked structure that includes a nonmagnetic film 28b, and ferromagnetic films 28 a and 28 c that sandwich the nonmagneticfilm 28 b, are antiferromagnetically coupled to each other, and have afixed magnetization direction, is used as the fixed magnetization layer28 of the MTJ, as shown in FIG. 4. In this synthetic structure, a fixedmagnetization layer 28, a free layer 26, and a tunnel barrier layer 27interposed in between form the MTJ. With the use of the syntheticantiferromagnetic stacked structure, a leakage magnetic field 29 fromthe ferromagnetic films 28 a and 28 c having fixed magnetization iscancelled at the edge portions of the ferromagnetic films 28 a and 28 c.Accordingly, the problem of a leakage magnetic field is not caused in aMTJ having ferromagnetic films with magnetization substantially parallelto the film plane. However, in a case where a syntheticantiferromagnetic stacked structure including vertical magnetizationfilms is used as the fixed magnetization layer of a MTJ, the filmthickness of the upper ferromagnetic film 18 c of the fixedmagnetization layer 18 of the synthetic antiferromagnetic stackedstructure is made greater than the film thickness of the lowerferromagnetic film 18 a, so as to reduce the leakage magnetic fieldapplied to the free layer 16 of the MTJ, as shown in FIG. 5. In FIG. 5,reference numeral 17 indicates a tunnel barrier layer, and referencenumeral 18 b′ indicates a nonmagnetic film. Even if the film thicknessof the upper ferromagnetic film 18 c is simply made greater than thefilm thickness of the lower ferromagnetic film 18 a, the leakagemagnetic field cannot be canceled, and shift adjustment cannot beperformed. Also, the adjacent transistors are adversely affected.

To counter this problem, a hard bias film 30 a is placed above thedevice-isolating insulating film 4 on the side of the source unit 15 a,a hard bias film 30 b above the device-isolating insulating film 4 onthe side of the drain unit 15 b, and a hard bias film 30 c is placedabove the gate electrode 10 in this embodiment, as shown in FIG. 3.

Since the hard bias films 30 a, 30 b, and 30 c are provided, the linesof magnetic force generated from the hard bias films 30 a, 30 b, and 30c (indicated by the broken lines), and the lines of magnetic forcegenerated from the leakage magnetic field from the fixed magnetizationlayer (indicated by the solid lines) cancel each other, as shown in FIG.3. Accordingly, the shift of the free layer 16 b located below the fixedmagnetization layer 18 b via the tunnel barrier layer 17 b can beadjusted, and a bias magnetic field is applied in such a direction as tofurther stabilize thermally the magnetization direction of the fixedmagnetization layer 18 a of the source unit 15 a provided on theopposite side of the gate 10. Since the hard bias films 30 a and 30 care placed above the device-isolating insulating films 4 serving as theboundaries of the adjacent spin MOSFETs, the influence of the leakagemagnetic filed from the MTJ on the adjacent spin MOSFETs can berestrained.

In this embodiment, the spin MOSFET is an n-MOSFET. More specifically,an n-p-n junction is formed in the semiconductor substrate. Thehigh-density n⁺-impurity diffusion layers 7 a and 7 b are formed on thesubstrate surfaces of the source region 5 a and the drain region 5 b.This structure can be formed by performing annealing by RTA (RapidThermal Annealing), after impurity doping is performed by ionimplantation in the same manner as in the formation of a conventionalMOS transistor. In a case where a p-MOSFET is formed, instead of then-MOSFET, a p-n-p junction should be formed with the use of impuritieshaving the opposite conductivity type from the above case, andhigh-density impurity diffusion layers (p⁺-impurity diffusion layers)should be formed on the substrate surfaces of the source region and thedrain region. By forming the n⁺-impurity diffusion layers or p⁺-impuritydiffusion layers, the interfacial resistance becomes lower, and a spinMOSFET that can perform spin injection writing at a higher speed can berealized. The replacement of an n-MOSFET with a p-MOSFET can also beapplied to the embodiments to be described below.

In this embodiment, to make the magnetization direction of the freelayer 16 b of the drain unit 15 b the same as the magnetizationdirection (parallel) of the fixed magnetization layer 18 b of the drainunit 15 b, spin-polarized electrons flow from the drain unit 15 b intothe source unit 15 a through the channel 8. Likewise, to make themagnetization direction of the free layer 16 b of the drain unit 15 bthe opposite from the magnetization direction (antiparallel) of thefixed magnetization layer 18 b of the drain unit 15 b, spin-polarizedelectrons flow from the drain unit 15 a into the source unit 15 b.

In the case of reading, current that does not change the magnetizationdirection of the free layer of the ferromagnetic stacked film of thedrain unit 15 b is supplied in the spin MOSFET. By doing so, apredetermined first voltage is applied to the gate 10, and current flowsbetween the source unit 15 a and the drain unit 15 b through the channel8. The electric resistance between the source unit 15 a and the drainunit 15 b is then measured. In this manner, reading can be performed.Also, writing can be performed by applying a predetermined secondvoltage, different from the first voltage in the case of reading, to thegate 10, and supplying current between the source unit 15 a and thedrain unit 15 b through the channel 8.

In this embodiment, the tunnel barrier layer 14 a is provided betweenthe source unit 15 a and the source region 5 a, and the tunnel barrierlayer 14 b is provided between the drain unit 15 b and the drain region5 b. However, the tunnel barrier layers 14 a and 14 b may be omitted. Insuch a case, a Schottky barrier is spontaneously formed at the interfacebetween the source unit 15 a and the source region 5 a, and at theinterface between the drain unit 15 b and the drain region 5 b. In thiscase, the adverse influence of a leakage magnetic field on the adjacenttransistors can be eliminated, and shift adjustment can be performed.

In the lateral spin MOSFET having the above structure, it has becomeapparent that the channel length dependence of the spin diffusion lengthin the channel region is greater than that in a spin MOSFET havingin-plane magnetization films. A spin MOSFET that has a magnetic materialwith a spin magnetization easy axis existing in the film plane, and aspin MOSFET that has a magnetic material with a magnetization easy axisextending in a direction perpendicular to the film plane are prepared,and the channel length dependence of the spin diffusion length in thechannel region is measured. As a result, it becomes apparent that thespin diffusion length becomes smaller as the channel length becomessmaller, as shown in FIG. 6. The ordinate axis in FIG. 6 indicates thesignal ΔV_(non-local) representing the spin diffusion length. FIG. 6shows the results of measurement carried out on the structure of thespin MOSFET structure that has vertical magnetization films according tothe third embodiment illustrated in FIG. 3, and a spin MOSFET structurethat has in-plane magnetization films and has the synthetic pinstructure shown in FIG. 4 as the fixed magnetization layer without ahard bias magnetic field. The materials used and the manufacture processare the same as those described below in Example 1. The causes of thedecrease in the spin diffusion length are considered to be the Hanleeffect caused by applying the leakage magnetic field generated from thehard bias films to the channel region, and be the spin relaxation thatis caused when the direction of the lines of magnetic force generatedfrom the leakage magnetic field becomes substantially perpendicular tothe direction of current (electron flow) flowing in the channel region.The spin diffusion length is evaluated by providing a nonmagnetic ohmicelectrode at a 100 μm distance, and measuring the sub spin currentobserved when the orientations of the magnetic bodies of thesource/drain units are changed into a parallel state or an antiparallelstate, with the potential of the ohmic electrode being the referencepotential. As can be seen from FIG. 6, if the channel length (theminimum distance between the source region and the drain region) is 0.25μm or less in a spin MOSFET having a lateral structure like the spinMOSFET of the third embodiment, the spin diffusion length becomes thesame as the spin diffusion length in a spin MOSFET structure havingin-plane magnetization films, and spin relaxation in the channel regiondue to a leakage magnetic field can be restrained.

Fourth Embodiment

FIG. 7 is a cross-sectional view of a spin MOSFET according to a fourthembodiment of the present invention. The spin MOSFET of the fourthembodiment is the same as the spin MOSFET of the third embodiment shownin FIG. 3, except that the tunnel barrier layers 14 a and 14 b areremoved, and the source unit 15 a is a MTJ that has a stacked structurein which a free layer 16 a, a tunnel barrier layer 17 a, and a fixedmagnetization layer 18 a are stacked in this order on the source region5 a.

Like the spin MOSFET of the third embodiment, the spin MOSFET of thisembodiment can eliminate the influence of a leakage magnetic field onadjacent transistors, and enables shift adjustment. The channel lengthis made 0.25 μm or less, so that spin relaxation in the channel regioncan be restrained as in the third embodiment.

In the spin MOSFET of this embodiment, the area of the film plane of theferromagnetic stacked film of the source unit 15 a is made larger thanthe area of the film plane of the ferromagnetic stacked film of thedrain unit 15 b. With this arrangement, the magnetization direction ofthe ferromagnetic layer 16 a of the source unit 15 a is not varied by awrite current, but the magnetization direction of the ferromagneticlayer 16 b of the drain unit 15 b is varied by a write current.Alternatively, the area of the film plane of the ferromagnetic stackedfilm of the source unit 15 a may be made smaller than the area of thefilm plane of the ferromagnetic stacked film of the drain unit 15 b.

With the above structure, only the free layer of the smaller-areaferromagnetic stacked film can be reversed at the time of spin injectionwriting. The write current I_(c) for spin injection writing isrepresented by the following equation:I _(c)=2eαMAt[H _(k) −H _(d)]/(hg)  (1)

where e represents the elementary charge, α represents the Gilbertdamping parameters, M represents magnetization, A represents the area ofthe film plane of the magnetic recording layer (the free layer with thesmaller area), t represents the film thickness of the magnetic recordinglayer (the free layer), H_(d) represents the demagnetizing field, H_(k)represents the anisotropic constant, and h represents the Planck'sconstant. Here, g represents the spin dependence efficiency g(θ) at theinterface between a fixed magnetization layer and a nonmagnetic layer,and is expressed by the following equation:g(θ)=[−4+(1+p)³(3+cos θ)/4p ^(3/2)]⁻¹  (2)

where p represents the spin polarization rate, and θ represents theangle formed by the magnetization directions of the two magnetic layers.Accordingly, the current that is spin-reversed by the free layer isproportional to the area of the film plane of the ferromagnetic stackedfilm. In this embodiment, the spin of the free layer of theferromagnetic stacked film of either the source unit or the drain unitis to be reversed. Therefore, the areas of the film planes of theferromagnetic stacked films of the source unit and the drain unit differfrom each other, and the magnetization of the free layer of theferromagnetic stacked film with the smaller area is always reversedfirst. The ratio between the areas of the film planes of the source unit15 a and the drain unit 15 b is preferably 1.1 or higher, or morepreferably 1.2 or higher, as will be shown in the below describedExamples.

As shown in FIGS. 8( a) and 8(b), the margin at the time of spininjection writing becomes wider, where one of the ferromagnetic stackedfilms of the source unit 15 a and the drain unit 15 b has a linearlysymmetric shape, and the other one has a linearly asymmetric shape. FIG.8( a) shows an example of the linearly symmetric shape, and FIG. 8( b)shows an example of the linearly asymmetric shape. However, the presentinvention is not limited to the examples illustrated in FIGS. 8( a) and8(b), and any linearly symmetric shape and linearly asymmetric shape maybe used. It has become apparent that no problems are caused as long asthe length ratio between the ferromagnetic stacked films is 1.1 orhigher. The linearly symmetric shape is used for the ferromagneticstacked film with the smaller-area film plane, and the linearlyasymmetric shape is used for the ferromagnetic stacked film with thelarger-area film plane.

Also, the spin directions of the free layer of the ferromagnetic stackedfilm with the larger area and the ferromagnetic layer in contact withthe nonmagnetic layer (the tunnel barrier layer) of the fixedmagnetization layer should preferably be parallel to each other. As canbe seen from Equations (1) and (2), the write current I_(c) in the casewhere the spin directions are parallel (cos θ=1) is larger than thewrite current I_(c) in the case where the spin directions areantiparallel (cos θ=−1), and therefore, a spin reversal in the case ofparallel spin directions is more difficult than a spin reversal in thecase of antiparallel spin directions. The spin direction in theferromagnetic stacked film with the larger area is always fixed, so asto avoid writing when the spin direction of the free layer of theferromagnetic stacked film with the smaller area is rewritten. At thispoint, the spin directions are made parallel to each other, so that theresistance of the spin MOSFET of this embodiment can be lowered, andspin injection writing can be performed at a higher speed.

As described above, in the case of writing, the magnetization directionof the free layer of the ferromagnetic stacked film with the larger areaof the source unit 15 a and the drain unit 15 b is not changed, but acurrent for reversing the magnetization direction of the free layer ofthe ferromagnetic stacked film with the smaller area is supplied to thespin MOSFET. In the case of reading, a current for not changing themagnetization directions of the free layers of the ferromagnetic stackedfilms of the source unit 15 a and the drain unit 15 b is supplied to thespin MOSFET. In doing so, the predetermined first voltage is applied tothe gate 10, and a current flows between the source unit 15 a and thedrain unit 15 b through the channel 8. Accordingly, reading can beperformed by measuring the electric resistance between the source unit15 a and the drain unit 15 b. Meanwhile, writing can be performed byapplying the predetermined second voltage, different from the voltagefor the reading, to the gate 10, and supplying a current between thesource unit 15 a and the drain unit 15 b through the channel 8.

As described above, in this embodiment, the areas of the film planes ofthe source unit 15 a and the drain unit 15 b differ from each other, butthe source unit 15 a and the drain unit 15 b have the same stackedstructures as each other. Accordingly, increases in production costs canbe restrained as much as possible.

In this embodiment, Schottky barriers are formed at the interfacebetween the source unit 15 a and the source region 5 a, and at theinterface between the drain unit 15 b and the drain region 5 b. However,the tunnel barrier layer 14 a may be provided between the source unit 15a and the source region 5 a, and the tunnel barrier layer 14 b may beprovided between the drain unit 15 b and the drain region 5 b. With thetunnel barrier layers 14 a and 14 b, the rate of magnetoresistancechange via the semiconductor becomes higher. Also, the tunnel barrierlayers 14 a and 14 b serve as the element diffusion barriers between thesemiconductor and the ferromagnetic layers. In a case where asemiconductor and a ferromagnetic layer are connected directly to eachother without a tunnel barrier layer, element diffusion is causedbetween the semiconductor and the ferromagnetic layer when thetemperature rises to 400° C. or higher. As a result, the spin signal viathe semiconductor might quickly deteriorate.

Fifth Embodiment

FIG. 9 is a cross-sectional view of a spin MOSFET according to a fifthembodiment of the present invention. The spin MOSFET of the fifthembodiment is the same as the spin MOSFET of the third embodiment shownin FIG. 3, except that the hard bias film 30 b is made closer to thefree layer 16 b of the drain unit 15 b, to increase the magnitude ofmagnetic force generated from the hard bias film 30 b. Therefore, thehard bias films 30 a and 30 c are also closer to the source unit 15 aand the gate electrode 10, respectively. Particularly, the gate includesthe hard bias film 30 c made of a magnetic material and accounts for ahalf of the gate electrode 10 of the third embodiment. Accordingly, thegate has a stacked structure that is formed of a gate insulating film 9,the gate electrode 10 placed on the gate insulating film 9, and the hardbias film 30 c placed on the gate electrode 10.

With this arrangement, the shift of the free layer 16 b located belowthe fixed magnetization layer via the tunnel barrier layer 17 b can beadjusted, and a bias magnetic field is applied in such a direction as tofurther stabilize thermally the magnetization direction of the fixedmagnetization layer 18 a placed on the opposite side of the gate fromthe drain unit 15 b. Thus, this structure is a preferred structure.

Like the third embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the thirdembodiment.

Sixth Embodiment

FIG. 10 is a cross-sectional view of a spin MOSFET according to a sixthembodiment of the present invention. The spin MOSFET of the sixthembodiment is the same as the spin MOSFET of the fourth embodiment shownin FIG. 7, except that the hard bias film 30 b is made closer to thefree layer 16 b of the drain unit 15 b, to increase the magnitude ofmagnetic force generated from the hard bias film 30 b. Therefore, thehard bias films 30 a and 30 c are also closer to the source unit 15 aand the gate electrode 10, respectively. Particularly, the gate includesthe hard bias film 30 c made of a magnetic material and accounts for ahalf of the gate electrode 10 of the second embodiment. Accordingly, thegate has a stacked structure that is formed of a gate insulating film 9,the gate electrode 10 placed on the gate insulating film 9, and the hardbias film 30 c placed on the gate electrode 10.

With this arrangement, the shift of the free layer 16 b located belowthe fixed magnetization layer via the tunnel barrier layer 17 b can beadjusted, and a bias magnetic field is applied in such a direction as tofurther stabilize thermally the magnetization direction of the fixedmagnetization layer 18 a placed on the opposite side of the gate fromthe drain unit 15 b. Thus, this structure is a preferred structure.

Like the fourth embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the fourthembodiment.

Seventh Embodiment

FIG. 11 is a cross-sectional view of a spin MOSFET according to aseventh embodiment of the present invention. The spin MOSFET of theseventh embodiment is the same as the spin MOSFET of the fifthembodiment shown in FIG. 9, except that the hard bias films 31 a and 31b each having a magnetization direction substantially perpendicular tothe film plane are provided on the nonmagnetic metal layers 20 a and 20b placed on the source unit 15 a and the drain unit 15 b, respectively.Accordingly, the hard bias films 31 a and 31 b are provided at thecontact positions of the source unit 15 a and the drain unit 15 b. Themagnetization directions of the hard bias films 31 a and 31 b are theopposite from (antiparallel to) the hard bias films 30 a, 30 b, and 30c.

Like the fifth embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the fifthembodiment. Also, a bias magnetic field is applied in such a directionas to further stabilize thermally the magnetization direction of thefixed magnetization layer 18 a placed on the opposite side of the gatefrom the drain unit 15 b. Thus, this structure is a preferred structure.

Further, in the third through sixth embodiments, a material such asCo—Pt, Co—Fe—Pt, CoPd, or Co—Fe—Pd is used as the material of the hardbias films. In the arrangement of the seventh embodiment, however, therestriction on the material of the hard bias films can be advantageouslyremoved. This is because the hard bias films 30 a, 30 b, 30 c, 31 a, and31 b are placed close to one another, and the magnetic force linesspontaneously cancel one another as shown in FIG. 11, as the hard biasfilms make one another “not hard”.

Eighth Embodiment

FIG. 12 is a cross-sectional view of a spin MOSFET according to aneighth embodiment of the present invention. The spin MOSFET of theeighth embodiment is the same as the spin MOSFET of the sixth embodimentshown in FIG. 10, except that the hard bias films 31 a and 31 b eachhaving a magnetization direction substantially perpendicular to the filmplane are provided on the nonmagnetic metal layers 20 a and 20 b placedon the source unit 15 a and the drain unit 15 b, respectively.Accordingly, the hard bias films 31 a and 31 b are provided at thecontact positions of the source unit 15 a and the drain unit 15 b. Themagnetization directions of the hard bias films 31 a and 31 b are theopposite from (antiparallel to) the hard bias films 30 a, 30 b, and 30c.

Like the sixth embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the sixthembodiment. Also, a bias magnetic field is applied in such a directionas to further stabilize thermally the magnetization direction of thefixed magnetization layer 18 a placed on the opposite side of the gatefrom the drain unit 15 b. Thus, this structure is a preferred structure.

Further, in the third through sixth embodiments, a material such asCo—Pt, Co—Fe—Pt, CoPd, or Co—Fe—Pd is used as the material of the hardbias films. In the arrangement of this embodiment, however, therestriction on the material of the hard bias films can be advantageouslyremoved. This is because the hard bias films 30 a, 30 b, 30 c, 31 a, and31 b are placed close to one another, and the magnetic force linesspontaneously cancel one another as shown in FIG. 12, as the hard biasfilms make one another “not hard”.

Ninth Embodiment

FIG. 13 is a cross-sectional view of a spin MOSFET according to a ninthembodiment of the present invention. The spin MOSFET of the ninthembodiment is the same as the spin MOSFET of the fifth embodiment shownin FIG. 9, except that the hard bias film 30 b is made closer to thefree layer 16 b of the drain unit 15 b so as to increase the influenceof the magnetic force lines generated from the hard bias film 30 b onthe free layer 16 b, and the hard bias film 30 c is made farther fromthe channel region 8 so as to reduce the influence of the magnetic forcelines generated from the hard bias film 30 c on the channel region 8.Accordingly, the hard bias films 30 a and 30 b are formed on thenonmagnetic metal films 20 a and 20 b, and the hard bias film 30 c isformed on the gate electrode 10 as high as the gate electrode 10 of thethird embodiment. Here, the magnetization directions of the hard biasfilms 30 a, 30 b, and 30 c are substantially parallel to one another,and are substantially antiparallel to the magnetization direction of theferromagnetic layer 18 b. With this magnetization arrangement, themagnetization of the ferromagnetic layer 16 a of the source unit 15 acan be stabilized, and the leakage magnetic field from the ferromagneticlayer 18 b toward the free layer 16 b in the drain 15 b can be reduced.Also, the influence of the leakage magnetic fields from the hard biasfilms 30 a, 30 b, and 30 c can be restrained in the channel 8.Accordingly, the shift of the free layer 16 b located below the drainunit 15 b via the tunnel barrier layer 17 b can be adjusted, and a biasmagnetic field is applied in such a direction as to further stabilizethermally the magnetization direction of the fixed magnetization layer18 a placed on the opposite side of the gate from the drain unit 15 b.Thus, this structure is a preferred structure. Also, since the influenceof the magnetic force lines from the bias film 30 c on the channelregion 8 is reduced, the adverse influence on the spin-polarizedelectrons passing through the channel region 8 can be reduced. The upperfaces of the hard bias films 30 a, 30 b, and 30 c are processed by CMPor the like, so as to exist in the same plane.

Like the third embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the fifthembodiment.

The structure of this embodiment may be applied to the seventh and theeighth embodiments illustrated in FIGS. 11 and 12.

Tenth Embodiment

FIG. 14 is a cross-sectional view of a spin MOSFET according to a tenthembodiment of the present invention. The spin MOSFET of the tenthembodiment is the same as the spin MOSFET of the third embodiment shownin FIG. 3, except that the fixed magnetization layer 18 a of the sourceunit 15 a is replaced with a fixed magnetization layer that is a stackedstructure in which a fixed magnetization film 18 a ₁, a nonmagnetic film18 a ₂, and a fixed magnetization film 18 a ₃ are stacked in this order.The magnetization directions of the fixed magnetization film 18 a ₁ andthe fixed magnetization film 18 a ₃ are substantially perpendicular tothe film plane, and are antiferromagnetically coupled to each other viathe nonmagnetic film 18 a ₂. In this embodiment, an antiferromagneticlayer 19 a is provided between the fixed magnetization layer 18 a of thesource unit 15 a and the nonmagnetic metal layer 20 a, and anantiferromagnetic layer 19 b is provided between the fixed magnetizationlayer 18 b of the drain unit 15 b and the nonmagnetic metal layer 20 b.The magnetization direction of the fixed magnetization layer 18 a of thesource unit 15 a becomes more stabilized by the antiferromagnetic layer19 a, and the magnetization direction of the fixed magnetization layer18 b of the drain unit 15 b becomes more stabilized by theantiferromagnetic layer 19 b. The nonmagnetic film 18 a ₂ is made of Ru,Rh, or Ir, or an alloy of any of those materials.

As described above, the fixed magnetization layer 18 a is a stackedstructure, and antiferromagnetic layers are stacked on the respectivefixed magnetization layers 18 a and 18 b. In this manner, themagnetization of the fixed magnetization layers 18 a and 18 b becomesmore stabilized. As for the antiferromagnetic layers, it is preferableto use PtMn, Ir—Mn, FeMn, Pt—Cr—Mn, Ni—Mn, or the like.

Like the third embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the thirdembodiment.

In this embodiment, the fixed magnetization layer 18 b of the drain unit15 b may be replaced with a fixed magnetization layer that is a stackedstructure formed of a fixed magnetization film, a nonmagnetic film, andanother fixed magnetization film.

Eleventh Embodiment

FIG. 15 is a cross-sectional view of a spin MOSFET according to aneleventh embodiment of the present invention. The spin MOSFET of theeleventh embodiment is the same as the spin MOSFET of the fourthembodiment shown in FIG. 7, except that the fixed magnetization layer 18a of the source unit 15 a is replaced with a fixed magnetization layerthat is a stacked structure in which a fixed magnetization film 18 a ₁,a nonmagnetic film 18 a ₂, and a fixed magnetization film 18 a ₃ arestacked in this order, and the fixed magnetization layer 18 b of thedrain unit 15 b is replaced with a fixed magnetization layer that is astacked structure in which a fixed magnetization film 18 b ₁, anonmagnetic film 18 b ₂, and a fixed magnetization film 18 b ₃ arestacked in this order. The magnetization directions of the fixedmagnetization film 18 a ₁ and the fixed magnetization film 18 a ₃ aresubstantially perpendicular to the film plane, and areantiferromagnetically coupled to each other via the nonmagnetic film 18a ₂. The magnetization directions of the fixed magnetization film 18 b ₁and the fixed magnetization film 18 b ₃ are substantially perpendicularto the film plane, and are antiferromagnetically coupled to each othervia the nonmagnetic film 18 b ₂. In this embodiment, anantiferromagnetic layer 19 a is provided between the fixed magnetizationlayer 18 a of the source unit 15 a and the nonmagnetic metal layer 20 a,and an antiferromagnetic layer 19 b is provided between the fixedmagnetization layer 18 b of the drain unit 15 b and the nonmagneticmetal layer 20 b. The magnetization direction of the fixed magnetizationlayer 18 a of the source unit 15 a becomes more stabilized by theantiferromagnetic layer 19 a, and the magnetization direction of thefixed magnetization layer 18 b of the drain unit 15 b becomes morestabilized by the antiferromagnetic layer 19 b. The nonmagnetic films 18a ₂ and 18 b ₂ are made of Ru, Rh, or Ir, or an alloy of any of thosematerials.

As described above, the fixed magnetization layers 18 a and 18 b arestacked structures, and antiferromagnetic layers are stacked on therespective fixed magnetization layers 18 a and 18 b. In this manner, themagnetization of the fixed magnetization layers 18 a and 18 b becomesmore stabilized. As for the antiferromagnetic layers, it is preferableto use PtMn, Ir—Mn, FeMn, Pt—Cr—Mn, Ni—Mn, or the like.

Like the fourth embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the fourthembodiment.

Twelfth Embodiment

FIG. 16 is a cross-sectional view of a spin MOSFET according to atwelfth embodiment of the present invention. The spin MOSFET of thetwelfth embodiment is the same as the spin MOSFET of the thirdembodiment shown in FIG. 3, except that a Heusler alloy layer 51 a isinserted between the tunnel barrier 14 a and the fixed magnetizationlayer 18 a of the source unit 15 a, a Heusler alloy layer 51 b ₁ isinserted between the tunnel barrier 14 b and the free layer 16 b, aHeusler alloy layer 51 b ₂ is inserted between the free layer 16 b andthe nonmagnetic layer 17 b, and a Heusler alloy layer 51 b ₃ is insertedbetween the nonmagnetic layer 17 b and the fixed magnetization layer 18b. As for the Heusler alloy layers, it is preferable to use a Co-basedfull-Heusler alloy having a high ferromagnetic transition temperature,such as CO₂FeAl_(x)Si_(1-x) or CO₂MnSi_(x)Al_(1-x).

As described above, a Heusler alloy layer is inserted between a tunnelbarrier and a vertical magnetization layer, and a Heusler alloy layer isinserted between a vertical magnetization layer and a tunnel barrier (anonmagnetic layer) in each vertical MTJ layer structure. With thisarrangement, the MR value via the channel region and the MR value ofeach vertical MTJ can be increased. By changing the relative thicknessratio between the Heusler alloy layers and the vertical magnetizationlayers, the magnetization easy-axis direction can be shifted to thevertical direction as a whole. In this embodiment, the tunnel barriers14 a and 14 b may be removed. The same effects as above can also beachieved, even if the Heusler alloy layers are replaced with CoFeBlayers.

Like the third embodiment, this embodiment can eliminate the influenceof a leakage magnetic field on adjacent transistors, and enables shiftadjustment. The channel length is made 0.25 μm or less, so that spinrelaxation in the channel region can be restrained as in the thirdembodiment.

The insertion of Heusler alloy layers or CoFeB layers may be applied tothe fourth through eleventh embodiments.

As described above, with the use of any of the structures of the thirdthrough twelfth embodiments, the shift of a free layer located below afixed magnetization layer via a tunnel barrier can be adjusted, and abias magnetic field is applied in such a direction as to furtherstabilize thermally the magnetization direction of the fixedmagnetization layer placed on the opposite side of the gate. Thus, apreferred structure can be obtained.

In the first through twelfth embodiments, examples of the materials offerromagnetic layers having a spin easy axis direction perpendicular tothe substrate in-plane preferably further include any one of a Fe—Pdlayer, a Fe—Pt layer, a Fe—Pd—Pt layer, a Gd—Fe—Co layer, a Tb—Fe—Colayer, a Dy—Fe—Co layer, a Gd—Co layer, a Tb—Co layer, a Dy—Co layer, amultilayer of (Gd(—Fe)Co/Co(—Fe)—B)_(n), a multilayer of(Tb(—Fe)Co/Co(—Fe)—B)_(n), a multilayer of (Dy(—Fe)Co/Co(—Fe)—B)_(n), aCo/Pd stacked film, a CoFe/Pd stacked film, a Co/Ni stacked film, aFe/Pd stacked film, and a Fe/Pt stacked film. Incidentally, n means anumber of layers in the multilayer.

In each of the spin MOSFETs of the first through ninth embodiments andthe twelfth embodiment, a stacked film that is formed of a ferromagneticstacked film having a nonmagnetic layer and vertical magnetization or isformed of an antiferromagnetic layer is stacked on at least one of theferromagnetic bodies of the source unit and the drain unit, and thenonmagnetic layer is made of Ru, Rh, or Ir, or an alloy of any of thosematerials. With this arrangement, the spin of the fixed magnetizationlayers becomes more stabilized, and the stability of the fixedmagnetization layers against heat becomes higher. Accordingly, even ifminiaturization is carried out through scaling, smaller spin MOSFETs canbe produced.

Each free layer (each magnetic recording layer) may have a stackedstructure formed of a ferromagnetic layer, a nonmagnetic layer, andanother ferromagnetic layer. If the nonmagnetic layers are made of Ru,Rh, or Ir, or an alloy of any of those materials in this case, thestability of each free layer becomes higher, and the stability of themagnetization easy axis at the time of writing becomes higher.Accordingly, even if miniaturization is carried out through scaling,smaller spin MOSFETs can be produced.

In each of the spin MOSFETs of the first through twelfth embodiments,the semiconductor substrate may be a Si substrate, a Ge substrate, SiGesubstrate, a GaAs substrate, or an InGaAs substrate. It is preferable toform a p-n-p junction or an n-p-n junction in each of those substrates,and form a high-density p⁺- or n⁺-region at the interface between aferromagnetic material and a semiconductor or the interface between thetunnel barrier and a semiconductor. With such a structure, theinterfacial resistance between a semiconductor and a tunnel barrier, ora Schottky barrier, can be reduced. Thus, a higher-speed spin MOSFET canbe realized. If n- or n⁺-doping is performed on each interface in a casewhere the semiconductor substrate is made of Ge, the interfacialresistance can be made lower by inserting an oxide or nitride such asGeOx or GeNx under the tunnel barriers. If the semiconductor substrateis a GaAs substrate, it is preferable to insert InGaAs or the like underthe tunnel barriers, so as to interfacial resistance.

In each of the first through twelfth embodiments, the tunnel barriersmay be made of MgO, SiO₂, SiNx, AlOx, AlNx, GeOx, GeNx, GaOx, arare-earth oxide, a rare-earth nitride, or a stacked film of one ofthose materials.

Although the spin MOSFETs according to the first through twelfthembodiments have been described in detail with reference to thedrawings, the drawings are merely schematic, and the sizes of thecomponents and the ratios in size among the components differ from thosein reality. Also, the size or proportion of each component drawn inseveral drawings might vary among those drawings.

If the electric conductivity between the magnetic material and thesemiconductor is high in each of the first through twelfth embodiments,the problem of conductance mismatching is caused, and the spinpolarization is saturated. As a result, spin injection into thesemiconductor cannot be performed. To solve this problem, ions areimplanted into a semiconductor such as Si, Ge, or GaAs, and a p/njunction should preferably be formed as in a conventional MOSFET. Indoing so, high-density ions are implanted into the interface between thesemiconductor and the magnetic material or the tunnel barrier interface,and (n⁺ or p⁺) ions should preferably be segregated. More specifically,in an n- or p-type MOSFET using a Si or Ge semiconductor substrate, ionimplantation of B (boron) is preferable as p-type impurity implantation,and ion implantation of P (phosphorus) or As (arsenic) is preferable asn-type impurity implantation. In the case of a GaAs substrate, ann-MOSFET is normally preferred, having higher mobility. In such a case,Si doping is normally performed. The accelerating voltage of the elementto be ion-implanted in n⁺-impurity regions or p⁺-impurity regions ismade as low as 20 KeV or lower, and high-density ion implantation shouldpreferably be performed. In the case of a Si substrate, the same elementcan be used for n-type impurity regions and n⁺-impurity regions, withouta problem being caused. In the case of a Ge substrate, however, P(phosphorus) or As (arsenic) should be used in n-type impurity regions,and S (sulfur) should more preferably be used in n⁺-impurity regions, soas to lower the resistance and obtain a high-speed device. After ionimplantation, RTA (Rapid Thermal Annealing) is performed in N₂. The RTAtemperature is in the range of 1000° C. to 1100° C. in the case of a Sisubstrate, and is in the range of 400° C. to 500° C. in the case of a Gesubstrate. In the case of a GaAs substrate, RTA is performed in As at atemperature between 300° C. and 600° C., or Si doping during filmformation is performed for growth. By any of those techniques, anexcellent spin MOSFET can be realized, and spin-dependent conduction isobserved.

EXAMPLES

The following is a description of examples of the present invention.

Example 1

As Example 1, the spin MOSFET of the third embodiment illustrated inFIG. 3 is formed. In the spin MOSFET of Example 1, the areas of theferromagnetic stacked films of the source unit 15 a and the drain unit15 b differ from each other. The semiconductor substrate is formed of animplanted Si substrate. The spin MOSFET including the source unit 15 aand the drain unit 15 b having ferromagnetic stacked films withdifferent areas from each other is formed in the following manner.First, as shown in FIG. 17, the source region 5 a and the drain region 5b are formed at a distance from each other in the semiconductorsubstrate 2. The gate insulating film 9 is formed on the portion of thesemiconductor substrate 2 located between the source region 5 a and thedrain region 5 b. The gate electrode 10 is formed on the gate insulatingfilm 9. Alternatively, the source region 5 a and the drain region 5 bmay be formed after the gate insulating film 9 and the gate electrode 10are formed. An interlayer insulating film 130 made of SiO₂ is thendeposited to cover the gate electrode 10. Holes 132 a and 132 b havingdifferent areas from each other are formed in the interlayer insulatingfilm 130. A ferromagnetic stacked film is deposited by high-pressure RFsputtering, to fill the holes 132 a and 132 b. The ferromagnetic stackedfilm deposited on the upper face of the interlayer insulating film isremoved by CMP (Chemical Mechanical Polishing). In this example, theholds in the interlayer insulating film 130 are formed one by one, so asto form different stacked films. Since non-local measurement is to bealso carried out, one of the device-isolating insulating films formed byLOCOS (Local Oxidation of Silicon) is located at a distance of 100 μmfrom the device, and the n⁺-regions 7 a and 7 b, and n-regions 6 a aredesigned to extend to the ohmic electrode located at a distance of 100μm. The formation of the gate electrode, ion implantation, and RTAprocessing are performed in the same manner as in the formation of aconventional MOSFET. Etchback is then performed after the formation ofthe interlayer insulating film 130, and flattening is performed on theinterlayer insulating film 130 to a certain degree, as shown in FIG. 17.The ferromagnetic stacked films of the source unit 15 a and the drainunit 15 b designed to have different areas but have the same aspectratio are formed, thereby filling the holes. After the hole-filling filmformation, CMP is performed. A SiO₂ film (not shown) is formed, and viasare opened. A wiring layer is then formed to fill the vias. Prior to theformation of the wiring, the areas of the source unit 15 a and the drainunit 15 b are measured by Shape SEM. The sizes of the holes 132 a and132 b for the source unit 15 a and the drain unit 15 b are 0.8 μm×0.8 μmand 0.3 μm×0.8 μm, which are the planned sizes of the source unit 15 aand the drain unit 15 b. The shapes of the actual holes are oval. Inthis example, the holes 132 a and 132 b for the source unit 15 a and thedrain unit 15 b may be the same as each other.

The stacked structures of the ferromagnetic stacked films 15 a and 15 bin this example are as follows.

The source unit 15 a has a structure formed by stacking layers on the Sisubstrate 2 in the following order: the tunnel barrier 14 a that is 0.8nm in film thickness and is made of MgO, the ferromagnetic layer 18 aformed of a 2.5-nm thick CoFeB layer and a 20-nm thick FePd layer, andthe nonmagnetic metal layer 20 a that is 50 nm in film thickness and ismade of Ta. The drain unit 15 b has a structure formed by stackinglayers on the Si substrate 2 in the following order: the tunnel barrier14 b that is 0.8 nm in film thickness and is made of MgO, the free layer16 b formed of a 2.5-nm thick CoFeB layer, a 10-nm thick FePd layer, anda 2.5-nm thick CoFeB layer, the tunnel barrier 17 b made of MgO, theferromagnetic layer 18 b formed of a 2.5-nm thick CoFeB layer and a20-nm thick FePd layer, and the nonmagnetic metal layer 20 b that is 50nm in film thickness and is made of Ta.

After that, the wiring layer is formed, and the hard bias films 30 a, 30b, and 30 c are formed at the locations shown in FIG. 3. The hard biasfilms 30 a, 30 b, and 30 c are made of CoPt.

As Comparative Example 1, a sample of a spin MOSFET is formed. This spinMOSFET is the same as the spin MOSFET of this example, except that thehard bias films 30 a, 30 b, and 30 c are removed. After the formation ofthe samples of this example and Comparative Example 1, 300° C. annealingis performed in a magnetic field for one hour.

After that, magnetic field writing is performed to form a spinantiparallel state and a spin parallel state by a difference in coerciveforce between the magnetic materials of the source/drain, with the gateelectrode being in an ON state. Experiments are then carried out to readthe rate of resistance change and measure the shift of each free layer.

FIGS. 18( a) and 18(b) show the results of the experiments. As shown inFIG. 18( b), shift adjustment cannot be performed in ComparativeExample 1. As shown in FIG. 18( a), shift adjustment can be performed byvirtue of the hard bias films in this example. Also, as illustrated inFIG. 6, samples with different channel lengths are formed, and spindiffusion lengths are evaluated. The evaluations are made by measuringthe sub spin current observed when the magnetization direction of thesource unit and the drain unit are parallel to or antiparallel to eachother. Since the channel length is 0.25 μm or less in this example, itis apparent that an effective structure is formed. The influence onadjacent cells is examined through magnetic field simulations. In thisexample, the magnetic layer of an adjacent spin MOSFET on the side of afree layer is a fixed magnetization layer. Accordingly, a bias magneticfield in such a direction as to further stabilize the spin is applied tothe fixed magnetization layer, without a problem being caused, asmentioned above.

Example 2

A sample of a spin MOSFET of Example 2 is the same as the spin MOSFET ofthe seventh embodiment illustrated in FIG. 11, except that the Sisubstrate is replaced with a Ge substrate. The manufacture method is thesame as that of Example 1.

The stacked structures of the ferromagnetic stacked films 15 a and 15 bin this example are as follows.

The source unit 15 a has a structure formed by stacking layers on the Gesubstrate in the following order: the tunnel barrier 14 a formed of a0.5-nm thick GeOx layer and a 0.5-nm thick MgO layer, the ferromagneticlayer 18 a formed of a 2.5-nm thick CoFeB layer and a 20-nm thick FePdlayer, and the nonmagnetic metal layer 20 a formed of a 50-nm thick Talayer. The drain unit 15 b has a structure formed by stacking layers onthe Ge substrate in the following order: the tunnel barrier 14 b formedof a 0.5-nm thick GeOx layer and a 0.5-nm thick MgO layer, the freelayer 16 b formed of a 2.5-nm thick CoFeB layer, a 10-nm thick FePdlayer, and a 2.5-nm thick CoFeB layer, the tunnel barrier 17 b made ofMgO, the fixed magnetization layer 18 b formed of a 2.5-nm thick CoFeBlayer and a 20-nm thick FePd layer, and the nonmagnetic metal layer 20 bformed of a 50-nm thick Ta layer.

After that, the wiring layer is formed, and the hard bias films 30 a, 30b, 30 c, 31 a, and 31 b are formed at the locations shown in FIG. 11.The hard bias films are made of CoPt. Also, a sample is formed asComparative Example 2, and this sample is the same as the sample of thisexample, except that the hard bias films are removed. After theformation of the samples of this example and Comparative Example 2, 270°C. annealing is performed in a magnetic field for one hour.

After that, magnetic field writing is performed to form a spinantiparallel state and a spin parallel state by a difference in coerciveforce between the magnetic materials of the source and drain, with thegate being in an ON state. Experiments are then carried out to read therate of resistance change and measure the shift of each free layer.

FIGS. 19( a) and 19(b) show the results of the experiments. As shown inFIG. 19( b), shift adjustment cannot be performed in Comparative Example2. As shown in FIG. 19( a), shift adjustment can be performed by virtueof the hard bias films in this example. This proves that the sameeffects can be achieved with different kinds of semiconductors. Theinfluence on adjacent cells is examined through magnetic fieldsimulations. In this example, the magnetic layer of an adjacent spinMOSFET on the side of a free layer is a fixed magnetization layer.Accordingly, a bias magnetic field in such a direction as to furtherstabilize the spin is applied to the fixed magnetization layer, withouta problem being caused, as mentioned above.

Example 3

A sample of a spin MOSFET of Example 3 is the same as the spin MOSFET ofthe fifth embodiment illustrated in FIG. 9, except that the Si substrateis replaced with a GaAs substrate. The manufacture method is the same asthat of Example 1.

The stacked structures of the ferromagnetic stacked films 15 a and 15 bin this example are as follows.

The source unit 15 a has a structure formed by stacking layers on theGaAs substrate in the following order: a 1.5-nm thick InGaAs layer, a0.7-nm thick GaOx layer, the ferromagnetic layer 18 a formed of a 2.5-nmthick CoFeB layer and a 20-nm thick FePd layer, and the nonmagneticmetal layer 20 a formed of a 50-nm thick Ta layer. The drain unit 15 bhas a structure formed by stacking layers on the GaAs substrate in thefollowing order: a 1.5-nm thick InGaAs layer, a 0.7-nm thick GaOx layer,the free layer 16 b formed of a 2.5-nm thick CoFeB layer, a 10-nm thickFePd layer, and a 2.5-nm thick CoFeB layer, the tunnel barrier 17 bformed of a 0.8-nm thick MgO layer, the fixed magnetization layer 18 bformed of a 2.5-nm thick CoFeB layer and a 20-nm thick FePd layer, andthe nonmagnetic metal layer 20 b formed of a 50-nm thick Ta layer.

After that, the wiring layer is formed, and the hard bias films 30 a, 30b, and 30 c are formed at the locations shown in FIG. 9. The hard biasfilms are made of CoPt. Also, a sample is formed as Comparative Example3, and this sample is the same as the sample of this example, exceptthat the hard bias films are removed. After the formation of the samplesof this example and Comparative Example 3, 300° C. annealing isperformed in a magnetic field for one hour.

After that, magnetic field writing is performed to form a spinantiparallel state and a spin parallel state by a difference in coerciveforce between the magnetic materials of the source and drain, with thegate being in an ON state. Experiments are then carried out to read therate of resistance change and measure the shift of each free layer. Theresults of the experiments are the same as those of Examples 1 and 2.Those results show that shift adjustment is enabled by the hard biasfilms. Those results also prove that the same effects can be achievedwith different kinds of semiconductors. The influence on adjacent cellsis examined through magnetic field simulations. In this example, themagnetic layer of an adjacent spin MOSFET on the side of a free layer isa fixed magnetization layer. Accordingly, a bias magnetic field in sucha direction as to further stabilize the spin is applied to the fixedmagnetization layer, without a problem being caused, as mentioned above.

Example 4

As Example 4, the spin MOSFET according to the second embodimentillustrated in FIG. 2 is formed on a Si substrate.

In this example, layers are stacked on the (001) Si substrate 61 in thefollowing order: the buried oxide film (BOX) 62; the GOI (Germanium OnInsulator) layer 63; the base layer 65 formed of a 0.6-nm thick MgOlayer, a 3-nm thick CoFeB layer, and a 10-nm TiN layer; the stacked film72 formed of a FePd layer and a Heusler alloy layer made ofCO₂FeAl_(0.5)Si_(0.5); the tunnel barrier 73 formed of a 0.8-nm thickMgO layer; the Ge channel layer 74; the tunnel barrier 77 formed of a0.8-nm thick MgO layer; the free layer 78 formed of a Heusler alloylayer made of CO₂FeAl_(0.5)Si_(0.5), a FePd layer, and a CoFeB layer;the tunnel barrier 80 formed of a 0.8-nm thick MgO layer; and themagnetic stacked film 82 formed of a CoFeB layer and a FePt layer. Thegate insulting film 90 a and the gate electrode 76 are formed on thesides of the channel layer 74.

The manufacture method according to this example is now described.First, a 150-nm thick Si_(0.9)Ge_(0.1) crystalline layer and a 5-nmthick Si cap layer are grown on a SOI substrate having a 20-nm thick SOIlayer by UHV-CVD (Ultra-High Vacuum Chemical Vapor Deposition) or MBE(Molecular Beam Epitaxy) or LP-CVD (Low Pressure Chemical VaporDeposition). At this point, each film thickness is less than thecritical film thickness at the growth temperature, and therefore,dislocation is not caused.

This wafer is then put into an oxidation furnace, and oxidation with theuse of an oxygen gas diluted 50% with nitrogen is performed at 1000° C.,until the thickness of the SiGe crystalline layer becomes 25 nm. Throughthis oxidation, Ge atoms are sufficiently diffused in the crystallinelayer interposed between a buried oxide film (the lower layer) and athermally-oxidized film (the upper layer). However, since an additionaloxide film is not formed, the Ge density is increased to 70% as thethickness of the crystalline layer becomes smaller in the course ofthermal oxidation. At this point, attention should be paid so that theoxidation temperature does not exceed the melting point of the SiGe withthe increased Ge density. To obtain a SeGe layer with Ge density of 70%as in this example, the oxidation temperature should be 1025° C. orlower at the end. To effectively shorten the oxidation time, thetemperature is first set at a high temperature that is not higher thanthe melting point according to the Ge density in the SiGe layer, and thetemperature is lowered gradually or stepwise.

After surface cleaning, a 0.6-nm thick MgO layer is formed, and a 3-nmthick CoFeB layer is formed on the MgO layer. Annealing is thenperformed to cause crystallization, and a 20-nm thick TiN layer isformed. The MgO layer, the CoFeB layer, and the TiN layer constitute thebase layer 65.

After surface cleaning, the stacked film 72 formed of a 20-nm thick FePdlayer and a 4-nm thick Heusler alloy layer made ofCO₂FeAl_(0.5)Si_(0.5), the tunnel barrier 73 made of MgO, the Ge channellayer 74, the tunnel barrier 77 made of MgO, the free layer formed of a3-nm thick Heusler alloy layer made of Co₂FeAl_(0.5)Si_(0.5), a 10-nmthick FePd layer, and a 2.5-nm thick CoFeB layer, the tunnel barrier 80formed of a 1.0-nm thick MgO layer, the magnetic stacked film 82consisting of: a 2.5-nm thick CoFeB layer; a magnetic layer formed of a20-nm thick FePt layer; a 0.9-nm thick Ru layer; and a 30-nm thick FePtlayer, and the 100-nm thick Ta layer 84 are stacked in this order bysputtering.

As for the crystalline material of the semiconductor layer 74 to be thechannel, it is possible to select a III-V compound semiconductor such asGaAs, other than Ge. If the base crystals are Ge, a III-V compound suchas GaAs can be stacked without dislocation or the like, having a latticeconstant relatively close to the lattice constant of Ge. It is alsopossible to select semiconductor crystals with different latticeconstants on purpose, and improve the mobility of the channel by causingdistortion in the channel portion. It is of course possible to selectSiGe or Si as the crystalline material of the semiconductor layer 74 tobe the channel.

After a protection film such as a deposited oxide film is inserted tothe surface, a photoresist pattern is formed, and RIE is performed onthe 100-nm Ta layer 84. With this Ta layer 84 serving as a hard mask,etching is performed on the upper magnetic stacked film and the Gecrystalline layer 74 to have island-like shapes by RIE. The etchingshould be stopped at the point when the Ge crystalline layer 74 isetched off, the etching of the base magnetic layer 72 is completed, andetching of the base layer 65 has just been started.

A thin insulating layer of 2 nm in thickness is then deposited on theentire surface by CVD. Here, SiO₂ is used. Part of this insulating layerturns into the gate insulating film. As the gate insulating film, it ispossible to use not only a Si oxide film (SiO₂), but also a Si nitridefilm (Si₃N₄), a Si oxynitride film (SiO_(x)N_(y)), or a high-dielectricgate insulating film made of Al₂O₃, Ta₂O₅, TiO₂, Ya₂O₃, or the like.Other than those materials, a Ge nitride film can also be used as thegate insulating film. A Ge nitride film can be obtained throughdeposition by CVD, direct nitridation of a Ge surface with the use of anammonia gas or a nitrogen gas, or the like.

After a polycrystalline Si layer of 20 nm to 25 nm in thickness for thegate electrode is deposited on the entire surface, phosphorus ionimplantation is performed with a dose amount of 5×10¹⁵ cm⁻². Annealingis further performed, to turn the polycrystalline Si layer into ahigh-density n-type layer. Alternatively, dopant may be added at thesame time as the deposition of the polycrystalline Si layer, and alow-resistance gate electrode may be formed by CVD. Also, a metal-basedgate electrode may be used. The gate electrode has such a shape as tosurround the channel portion of the vertical transistor. Thepolycrystalline Si layer deposited on the source/drain portions of theuppermost layer is removed by flattening. The polycrystalline Si layeris then removed by photolithography, remaining only at the portionsurrounding the gate.

Lastly, the lower and upper magnetic layers are formed, and an openingfor the polycrystalline Si electrode is formed. The electrode is thenformed, thereby completing the device. After the formation of thesample, annealing is performed at 270° C. in a magnetic field for onehour. After that, magnetic field writing is performed to form a spinantiparallel state and a spin parallel state by a difference in coerciveforce between the magnetic materials of the source and drain, with thegate being in an ON state. An experiment is then carried out to read therate of resistance change and measure the shift of each free layer. Theresults of the experiment show that shift adjustment is enabled, as inExamples 1 through 3. The influence on adjacent cells is examinedthrough magnetic field simulations. In this example, even where thechannel length is 20 nm, influence on adjacent cells is eliminated if asynthetic structure is used for the fixed magnetization layer of thevertical FET structure illustrated in the first and second embodiments.

FIG. 20 shows the channel length dependence of the output voltagedifference ΔVlocal between a case where the spins in magnetic bodies arein a parallel state and a case where the spins in the magnetic bodiesare in an antiparallel state in samples of vertical FET structures(vertical structures) in which the channel length varies from 20 nm to500 nm. In FIG. 20, a case where a lateral FET (a lateral structure) hasa Ge layer as the channel and has in-plane magnetization films in themagnetic material, and a case where a lateral FET has verticalmagnetization films are also plotted. The magnetic materials andstructures are the same as those explained with reference to FIG. 6. Ascan be seen from FIG. 20, the direction of the leakage magnetic field isparallel to the current flowing direction in the vertical FET structure.Accordingly, spin relaxation due to a Hanle effect is restrained, andpreferred results are achieved in the vertical FET structure.

Next, an embodiment concerning a reconfigurable logic circuit formed ofspin MOSFETs of the above embodiments and examples is described.

Thirteenth Embodiment

A reconfigurable logic circuit according to a thirteenth embodiment ofthe present invention is described. The reconfigurable logic circuit ofthis embodiment is a logic circuit formed of spin MOSFETs according toone of the first through twelfth embodiments and Examples 1 through 4.

First, a circuit structure actually used in a reconfigurable logiccircuit is briefly described. In a case where a reconfigurable logiccircuit is actually formed of spin MOSFETs, two MOSFETs (MOSFET 1 andMOSFET 2) preferably have a common floating gate.

If an AND circuit and an OR circuit can be formed, all circuits such asa NOR circuit and an exclusive OR circuit can be formed. Therefore, FIG.21 shows only an AND circuit and an OR circuit. As shown in FIG. 21, thereconfigurable logic circuit of this embodiment basically uses two spinMOSFETs 150 and 152 of one of the first through twelfth embodiments andExamples 1 through 4. In each of the spin MOSFETs 150 and 152, afloating gate (not shown) and an interelectrode insulating film areprovided between the gate insulating film 9 and the gate electrode 10.The spin MOSFET 150 is a p-type MOSFET, or a MOSFET formed in an n-typewell region (not shown) of a p-type semiconductor substrate. The spinMOSFET 152 is an n-type MOSFET, or a MOSFET formed in a p-typesemiconductor region of the p-type semiconductor substrate. The floatinggates of the MOSFETs 150 and 152 are connected to each other to form thecommon floating gate. The source of the MOSFET 150 is connected to apower supply Vinp, and the source of the MOSFET 152 is grounded. Anoutput V1 from the common-connected node is input to an inverter 160,and an output of this inverter 160 is an output Vout of the logiccircuit of this embodiment.

In this manner, an AND circuit and an OR circuit can be formed. As shownin FIG. 22, in a case where the floating gate voltage Vfg is ½ of thesum of the gate input A of the MOSFET 150 and the gate input B of theMOSFET 152, the output voltage Y obtained when the spin moments of theferromagnetic layers of the source and drain closer to the semiconductorsubstrate 2 are in a parallel (P) state or an antiparallel (AP) state ischanged to “1” or “0”. In this embodiment, the spin moments in theMOSFET 150 are always in a parallel state.

FIG. 23 shows values of the floating gate potential Vfg, the potentialV1 of the common-connected node between the MOSFETs 150 and 152, and theoutput Vout of the logic circuit, with respect to the inputs A and B ofthe gate electrodes of the MOSFETs 150 and 152 in a case where the spinmoments in the ferromagnetic layers of the source and drain closer tothe semiconductor substrate 2 in the MOSFET 152 are in an AP(antiparallel) state in the logic circuit of this embodiment. FIG. 24shows values of the floating gate potential Vfg, the potential V1 of thecommon-connected node between the MOSFETs 150 and 152, and the outputVout of the logic circuit, with respect to the inputs A and B of thegate electrodes of the MOSFETs 150 and 152 in a case where the spinmoments in the ferromagnetic layers of the source and drain closer tothe semiconductor substrate 2 in the MOSFET 152 are in a P (parallel)state. As shown in FIGS. 23 and 24, the logic circuit becomes an ANDcircuit when the spin moments in the ferromagnetic layers of the sourceand drain closer to the semiconductor substrate 2 in the MOSFET 152 arein an antiparallel state. The logic circuit becomes an OR circuit whenthe spin moments in the ferromagnetic layers of the source and draincloser to the semiconductor substrate 2 in the MOSFET 152 are in aparallel state. Accordingly, by changing the spin moment in theferromagnetic layer of the drain unit and performing reprogramming, alogic circuit can be formed without reconstruction, and a reconfigurablelogic circuit can be obtained.

In the case of an AND circuit and an OR circuit, all the transistors maybe spin MOSFETs, but it is possible to use conventional MOSFETs for someof the transistors. As shown in FIG. 25, one (the MOSFET 152, forexample) of the two transistors may be a spin MOSFET of one of the firstthrough twelfth embodiments and Example 1 through 4, and the other onemay be a conventional p-MOSFET 154 not including a magnetic material. Insuch a case, the same effect as above can be achieved by controlling thespin moments in the ferromagnetic layers of the source and drain closerto the semiconductor substrate 2 in the spin MOSFET 152 between aparallel state and an antiparallel state.

Also, as shown in FIG. 26, the spin moments in the ferromagnetic layersof the source and drain closer to the semiconductor substrate 2 in thep-type MOSFET 150 can be controlled between a parallel state and anantiparallel state by switching the connections of the n-type MOSFET 152and the p-type MOSFET 150 without the use of the inverter 160. In thismanner, the same effect as above can be achieved.

When used as the logic circuit, the structure according to thisembodiment further includes a gate voltage control circuit for readinginformation from spin MOSFETs, a sense current control device circuitfor controlling sense current, a write current control circuit, adriver, and a sinker.

The reconfigurable logic circuit of this embodiment is merely onespecific example, and reconfigurable logic circuits that can be formedof spin MOSFETs of one of the first through twelfth embodiments andExamples 1 through 4 are not limited to the reconfigurable logic circuitof this embodiment.

Where any of the above spin MOSFETs is used, a device having high MR andlow resistance can be realized, and accordingly, a reconfigurable logiccircuit can be realized.

In accordance with any of the embodiments of the present invention, evenwhen vertical magnetization films are used for the ferromagneticmaterial of a MTJ in the source/drain regions of a spin MOSFET,influence of a leakage magnetic field on adjacent transistors can berestrained, shift adjustment can be performed, and spin relaxation inthe channel region can be restrained.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcepts as defined by the appended claims and their equivalents.

1. A spin MOSFET, comprising: a source unit and a drain unit provided ata distance from each other on a semiconductor substrate, one of thesource unit and the drain unit comprising a first ferromagnetic stackedfilm that includes a first ferromagnetic layer having a fixedmagnetization direction perpendicular to a film plane, the other one ofthe source unit and the drain unit comprising a second ferromagneticstacked film that includes a second ferromagnetic layer having avariable magnetization direction perpendicular to a film plane, a firsttunnel barrier provided on the second ferromagnetic layer, and a thirdferromagnetic layer provided on the first tunnel barrier and having afixed magnetization direction perpendicular to a film plane andantiparallel to the magnetization direction of the first ferromagneticlayer; a gate insulating film provided on a portion of the semiconductorsubstrate, the portion being located between the source unit and thedrain unit; a gate electrode provided on the gate insulating film; afirst hard bias film provided above the gate electrode; a second hardbias film provided above a portion of the semiconductor substrate, theportion being located on a first side of the source unit, the first sidebeing opposite to a second side on which the first hard bias film isdisposed; and a third hard bias film provided above a portion of thesemiconductor substrate, the portion being located on a third side ofthe drain unit, the third side being opposite to a fourth side on whichthe first hard bias film is disposed, magnetization directions of thefirst through third hard bias films being parallel to the magnetizationdirection of the third ferromagnetic layer.
 2. The spin MOSFET accordingto claim 1, further comprising a stacked film that includes anonmagnetic layer, a ferromagnetic layer having vertical magnetization,and an antiferromagnetic layer stacked in this order is formed on atleast one of the first and second ferromagnetic stacked films, and thenonmagnetic layer includes at least one of Ru, Rh, and Ir.
 3. The spinMOSFET according to claim 1, wherein the first and second ferromagneticstacked films differ in film plane area or film plane shape.
 4. The spinMOSFET according to claim 1, wherein a source region and a drain regionare provided by introducing impurities into portions of thesemiconductor substrate, the portions being located below the sourceunit and the drain unit, each of the source region and the drain regionhas a first impurity region and a second impurity region providedbetween the first impurity region and the source unit or the drain unit,the second impurity region having higher impurity density than the firstimpurity region, and the shortest distance between the source region andthe drain region is 0.25 μm or less.
 5. The spin MOSFET according toclaim 1, wherein the first through third ferromagnetic layers eachinclude one of a Fe—Pd layer, a Fe—Pt layer, a Fe—Pd—Pt layer, aGd—Fe—Co layer, a Tb—Fe—Co layer, a Dy—Fe—Co layer, a Gd—Co layer, aTb—Co layer, a Dy—Co layer, a multilayer of (Gd(—Fe)Co/Co(—Fe)—B)_(n), amultilayer of (Tb(—Fe)Co/Co(—Fe)—B)_(n), a multilayer of(Dy(—Fe)Co/Co(—Fe)—B)_(n), a Co/Pd stacked film, a CoFe/Pd stacked film,a Co/Ni stacked film, a Fe/Pd stacked film, and a Fe/Pt stacked film. 6.A spin MOSFET, comprising: a source unit and a drain unit provided at adistance from each other on a semiconductor substrate, one of the sourceunit and the drain unit comprising a first ferromagnetic stacked filmthat includes a first ferromagnetic layer having a fixed magnetizationdirection perpendicular to a film plane, the other one of the sourceunit and the drain unit comprising a second ferromagnetic stacked filmthat includes a second ferromagnetic layer having a variablemagnetization direction perpendicular to a film plane, a first tunnelbarrier provided on the second ferromagnetic layer, and a thirdferromagnetic layer provided on the first tunnel barrier and having afixed magnetization direction perpendicular to a film plane andantiparallel to the magnetization direction of the first ferromagneticlayer; a gate insulating film provided on a portion of the semiconductorsubstrate, the portion being located between the source unit and thedrain unit; a gate electrode provided on the gate insulating film; afirst hard bias film provided on the gate electrode; a second hard biasfilm provided on the source unit; and a third hard bias film provided onthe drain unit, magnetization directions of the first through third hardbias films being substantially antiparallel to the magnetizationdirection of the third ferromagnetic layer.
 7. The spin MOSFET accordingto claim 6, further comprising a stacked film that includes anonmagnetic layer, a ferromagnetic layer having vertical magnetization,and an antiferromagnetic layer stacked in this order is formed on atleast one of the first and second ferromagnetic stacked films, and thenonmagnetic layer includes at least one of Ru, Rh, and Ir.
 8. The spinMOSFET according to claim 6, wherein the first and second ferromagneticstacked films differ in film plane area or film plane shape.
 9. The spinMOSFET according to claim 6, wherein a source region and a drain regionare provided by introducing impurities into portions of thesemiconductor substrate, the portions being located below the sourceunit and the drain unit, each of the source region and the drain regionhas a first impurity region and a second impurity region providedbetween the first impurity region and the source unit or the drain unit,the second impurity region having higher impurity density than the firstimpurity region, and the shortest distance between the source region andthe drain region is 0.25 μm or less.
 10. The spin MOSFET according toclaim 6, wherein the first through third ferromagnetic layers eachinclude one of a Fe—Pd layer, a Fe—Pt layer, a Fe—Pd—Pt layer, aGd—Fe—Co layer, a Tb—Fe—Co layer, a Dy—Fe—Co layer, a Gd—Co layer, aTb—Co layer, a Dy—Co layer, a multilayer of (Gd(—Fe)Co/Co(—Fe)—B)_(n), amultilayer of (Tb(—Fe)Co/Co(—Fe)—B)_(n), a multilayer of(Dy(—Fe)Co/Co(—Fe)—B)_(n), a Co/Pd stacked film, a CoFe/Pd stacked film,a Co/Ni stacked film, a Fe/Pd stacked film, and a Fe/Pt stacked film.11. A reconfigurable logic circuit, comprising two field effecttransistors, at least one of the two field effect transistors being thespin MOSFET according to claim 1, the two field effect transistorshaving a common floating gate.